In many countries outside the United States, telephone line circuits frequently include tone burst generators for use with periodic pulse metering (PPM) telephone lines. The generators transmit tone bursts to telephone station equipment for use in incrementing call cost meters and in controlling coin collection at public telephones. It is important to control the tone burst envelope to avoid in-band noise resulting in audible clicking on the line. Since it is necessary to have one generator per PPM line, the unit cost of such generators is a very significant factor.
In one prior art arrangement, tone bursts are generated by turning an oscillator filter unit ON and OFF. However, since the oscillator filter units include inductors, the unit size and cost are negative factors.
In a second prior an arrangement, tone bursts are generated using a four-quadrant linear multiplier circuit. The circuit cost is again too high. In addition, a reduction in the idle channel noise--the magnitude of the tone which persists after a tone burst is over--would be desirable.